The present invention relates to a slip-controlled brake system for motor vehicles which comprises a braking pressure generator fed from an auxiliary energy source, to which generator the wheel brakes of each front wheel and the respective diagonally opposite rear wheel are connected by way of two hydraulically separated pressure medium circuits. Inlet valves are inserted into the circuits which normally in their initial position are switched to open passage. Return flow conduits are provided which connect the wheel brakes with the pressure compensation reservoir and into which outlet valves are inserted which are closed in their initial position. The brake system further comprises wheel sensors for determination of the rotational behavior of the wheels and electric circuits for processing the sensor signals and generating braking pressure control signals which can be transmitted to the inlet and outlet valves.
The locking of the vehicle wheels during a braking operation can be prevented in brake systems operated hydraulically or by compressed air by means of so-called inlet and outlet valves which are inserted into the pressure medium path towards the wheel brakes and/or into a return flow conduit or into a pressure relief path and by which, as soon as the delay and/or wheel slip condition exceeds a certain value, a further braking pressure increase can be prevented or the braking pressure reduced. Systems of this type are known in numerous variations.
Optimal braking behavior, (i.e. high driving or directional stability while achieving a short stopping distance) is to be expected at each vehicle wheel when the braking pressure is controlled individually. However, since the number of components required, including control and regulating circuits, monitoring and safety circuits, etc., is very large and a convenient restriction to two or three regulating channels in connection with certain selection criteria, such as select-low, select-high, and with balanced control of the braking pressure in several wheel brakes will lead to relatively low losses, it is expedient to avoid individual control of all wheels.
Therefore, double-circuit hydraulic brake systems of the type described have been developed and described, by which upon recognition of a locking hazard the braking pressure can be varied in phase (i.e. concurrently for both associated wheels) by means of a common pair of inlet and outlet valves. If in this process the pressure in each regulating channel is determined according to the select-low principle by the wheel with the worst road contact, a longer stopping distance must be tolerated.
A further known measure is to insert in such brake systems an additional two-way/two-position directional control valve in the pressure medium path towards the wheel brake at the rear wheel. Accordingly, when the rear wheel is relieved as a result of the dynamic shift of the weight on the axle upon switching over of this valve a locking of the relieved rear wheel is prevented and at the same time a further braking pressure increase is provided at the front wheel (DE disclosure document No. 31 36 616). In situtations where the use of the braking effect of the rear wheels is essential, in particular in case of low adherence value, this known measure offers no improvement.
It has also been proposed to normally derive the control input for determination of the braking pressure in the diagonal from the front wheel, but to effect a switching-over in certain situations according to pre-determined criteria so that the rear wheel instead of the front will temporarily determine the braking pressure (DE-P No. 33 14 802.3). In other situations, joint control of the braking pressure of both wheels connected to a pressure medium circuit as compared to systems with three or four control channels is nevertheless disadvantageous.
It is also known to apply the selection criteria "select-low/select-high" determining control of the pressure in both circuits temporarily to specific configurations (EP No. 51 801), in order to reduce the number of control channels in a diagonally arranged double-circuit brake system and to provide a single pressure modulator per pressure medium circuit and, for better adaptation of the braking pressure regulation to the respective situation. Here, a locking of the wheel bearing the lead load is accepted in favor of a short stopping distance.
The object of the present invention is to overcome the disadvantages described above, to improve the controllability of a brake system of the initially mentioned type with at most minor additional expense, and in particular to increase substantially the braking effect at a low coefficient of friction without adversely influencing the braking effect at a high coefficient of friction.